The ionosphere at an altitude of 70 kilometers acts like a giant mirror for radio waves. Ground antennas constantly beam signals into it and listen for the reflection. When a solar flare occurs on the Sun, a stream of X-rays reaches Earth in 8 minutes and compresses the lower layer of the ionosphere. It's like tilting a mirror slightly — the reflected beam shifts in phase (the arrival time of the wave). Sensitive receivers notice this shift and instantly alert to powerful M-class and X-class flares, which are dangerous for satellites and power grids.
The system combines data from multiple transmitters, as if we were looking at the mirror from different angles, and using spectrometric assessment determines the strength of the burst. Ground antennas are cheap and independent of satellites, which can delay data. In the future, this will become part of a global defense against space weather.
The very low frequency waves used in this method can travel through water — which is why they are used to communicate with submarines at depth.
Now we're not just reading about space weather in sci-fi: we're listening to solar storms through the ionosphere's radio mirror.